Gas turbines suffer a loss in efficiency because of a portion of the gas passing between the turbine blades and the casing, rather than passing through the blades where the gases do work. Various schemes have been used to decrease this leakage. One such scheme takes advantage of cooling air which is passing though a pocket within the blade by discharging the spent cooling through directed openings in the blade tip. The air is directed at a low angle with respect to the surface of the blade tip in a direction against the flow of gas.
At locations of the turbine blade where the blade is relatively wide, such a low angle hole may be drilled and extend far enough to reach the internal pocket. However, at some locations on the blades, particularly near the trailing edge, the blade is relatively narrow. A straight low angle hole will not contact the air pocket.
The purpose of these openings is not just to discharge gas flow into the linkage space, but to project the air at the low angle countering the leakage flow. Accordingly, the drilled hole should have appropriate peripheral surface to properly direct the flow of air.
Electrode discharge machine drilling has been used for drilling various cooling air holes in turbine blades. It is required that these holes be small and precise in order to properly distribute the airflow through the multiple holes arranged in parallel Such a process of machining is shown in U.S. Pat. No. 4,762,464, issued Aug. 9, 1988 to Vertz et al. Such conventional EDM drilling may be used where the straight line projection of the hole to be drilled intersects the internal pocket. The problem develops, however, where because of the narrowness of the blade the extension of the drill hole does not reach the pocket.